Silicon surface texturing method for reducing surface reflectance

ABSTRACT

A method of texturing a surface of a crystalline silicon substrate is provided. The method includes immersing a crystalline silicon substrate into an aqueous alkaline etchant solution to form a pyramid shaped textured surface, with (111) faces exposed, on the crystalline silicon substrate. The aqueous alkaline etchant solution employed in the method of the present disclosure includes an alkaline component and a nanoparticle slurry component. Specifically, the aqueous alkaline etchant solution of the present disclosure includes 0.5 weight percent to 5 weight percent of an alkaline component and from 0.1 weight percent to 5 weight percent of a nanoparticle slurry on a dry basis.

BACKGROUND

The present disclosure relates to crystalline silicon solar cellmanufacturing, and more particularly, to a method for texturing acrystalline silicon substrate utilizing an aqueous alkaline etchantsolution which includes a nanoparticle slurry as one of the componentsthereof.

A photovoltaic device is a device that converts the energy of incidentphotons to electromotive force (e.m.f.). Typical photovoltaic devicesinclude solar cells, which are configured to convert the energy in theelectromagnetic radiation from the Sun to electric energy.

In a typical solar cell, single-crystalline silicon is generally used asone of the components of the cell. In such applications, thesingle-crystalline silicon needs to have a non-planar surface to improvelight capture. Typically, the non-planar surface has concave and convexpatterns with a minute pyramid (i.e., square pyramid) shape. In suchsolar cells, the light reflected from one spot impinges again to anotherspot on the surface of the crystalline solar cell by virtue of the‘textured’ surface, penetrating into the solar cell to be effectivelyabsorbed in the solar cell. Although a portion of the impinging lightthat has not been fully absorbed, but arrives at the back face of thesolar cell, is reflected back to the surface again, that portion ofimpinging light can be reflected again at the surface comprising steeplyinclined pyramidal surfaces, thereby confining the light in the solarcell to improve absorption of light and to enhance power generation.

In conventional single-crystalline silicon solar cells, the texturedstructure is formed by immersing the exposed (100) face of asingle-crystalline silicon wafer into an alkaline solution, i.e., sodiumhydroxide (NaOH) or potassium hydroxide (KOH), which may also include 5to 30% by volume of isopropyl alcohol and/or 0.3 to 1.5% by weight ofsilicon. The silicon is etched into the bath as a bath conditioner.

The etching rate in anisotropic etchants of the kind described abovedepends on the crystallographic orientation of the silicon surface beingetched. The etching rate on the (111) face is significantly lower thanthe other crystallographic orientations. Accordingly, the (111) facewith the slowest etching rate is advantageously left on the surface.Since this (111) face is inclined by about 54 degree against the (100)face, pyramidal projections constituted of the (111) face and itsequivalent faces are formed. The pyramid size and density depends on theKOH or NaOH concentration, the amount of silicon already dissolved inthe bath, and additive such as isopropyl alcohol.

SUMMARY

In one embodiment, an aqueous alkaline etchant solution is provided thatcan be used to texture a surface of a crystalline silicon substrate. Inthis embodiment, the aqueous alkaline etchant solution includes 0.5weight percent to 5 weight percent of an alkaline component and from 0.1weight percent to 5 weight percent of a nanoparticle slurry. No alcohol,surfactant, or pre-etched conditioning silicon is necessary to achievecomplete pyramid coverage and low reflectance in the present disclosure.

In another embodiment, a method of texturing a surface of a crystallinesilicon substrate is provided. Specifically, the method of the presentdisclosure includes immersing a crystalline silicon substrate into anaqueous alkaline etchant solution to form a pyramid shaped texturedsurface, with (111) faces exposed, on the crystalline silicon substrate.The aqueous alkaline etchant solution employed in the method of thepresent disclosure includes an alkaline component and a nanoparticleslurry. Again, no alcohol, surfactant, or pre-etched conditioningsilicon is necessary to achieve complete pyramid coverage and lowreflectance in the present disclosure.

When the aqueous alkaline etchant solution of the present disclosure isused to texture a crystalline silicon surface, the aqueous alkalinesolution provides a substantially improved textured crystalline Sisurface, with an increased density of smaller-sized pyramids compared toa KOH-only process. The improved morphology of the textured surface thatis achieved by utilizing the aqueous alkaline etchant solution of thepresent disclosure leads to a significant decrease in the measuredreflectance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation (through a cross sectional view)illustrating a crystalline silicon substrate that can be employed in oneembodiment of the present disclosure.

FIG. 2 is a pictorial representation (through a cross sectional view)illustrating the structure shown in FIG. 1 after immersing thesingle-crystalline silicon substrate in an etchant solution inaccordance with the present disclosure.

FIG. 3 is a scanning electron microscopic (SEM) photograph at amagnification of 2.00 KX showing the textured surface of a saw damagedetched (SDE) single-crystalline silicon substrate after immersing thesame in an aqueous alkaline etchant solution including a nanoparticleslurry in accordance with Example 1 which represents an embodiment ofthe present disclosure.

FIG. 4 is a scanning electron microscopic (SEM) photograph at amagnification of 2.00 KX showing the textured surface of an as-cutsingle-crystalline silicon substrate after immersing the same in anaqueous alkaline etchant solution including a nanoparticle slurry inaccordance with Example 2 which represents another embodiment of thepresent disclosure.

FIG. 5 is a scanning electron microscopic (SEM) photograph at amagnification of 2.00 KX showing the textured surface of an as-cutsingle-crystalline silicon substrate after immersing the same in a priorart aqueous alkaline etchant solution including KOH and silicon asdescribed in Comparative Example 1.

FIG. 6 is a scanning electron microscopic (SEM) photograph at amagnification of 2.00 KX showing the textured surface of an as-cutsingle-crystalline silicon substrate after immersing the same in a priorart aqueous alkaline etchant solution including only KOH with no Si ornanoparticle slurry as described in Comparative Example 2.

DETAILED DESCRIPTION

The present disclosure, which provides a method of texturing acrystalline silicon surface using an aqueous alkaline etchant solutionincluding a nanoparticle slurry, will now be described in greater detailby referring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings are provided forillustrative purposes only and are not drawn to scale.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to illustrate the presentdisclosure. However, it will be appreciated by one of ordinary skill inthe art that various embodiments of the present disclosure may bepracticed without these, or with other, specific details. In otherinstances, well-known structures or processing steps have not beendescribed in detail in order to avoid obscuring the various embodimentsof the present disclosure.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

Texturing of silicon surfaces is one of the necessary first steps in thefabrication of crystalline Si-based solar cells to achieve minimal waferreflectance and increased light absorption. The present disclosureprovides a wet texturing method that generates crystalline Si wafersurfaces with complete pyramid coverage and improved reflectanceresponse, significantly lower reflectance than those processed in aKOH-only solution and comparable, if not lower, to those processed in aKOH/IPA or a KOH/Si mixture solution. In addition, the method of thepresent disclosure employs a stable chemistry with no flammablecomponent.

The method of the present disclosure utilizes an aqueous alkalineetchant solution including a nanoparticle slurry as one of thecomponents of the etchant solution to texture a surface of a crystallinesilicon substrate. The use of the aqueous alkaline etchant solution ofthe present disclosure provides a substantially improved texturedcrystalline silicon surface, with an increased density of smaller-sizedpyramids compared to a KOH-only process. The improved morphology of thetextured surface that is achieved by the present disclosure leads to asignificant improvement in the measured reflectance. For example, basedon measurements performed by the applicants of the present disclosure, acrystalline Si surface textured with a KOH-only bath is twice asreflective as a similar surface textured by the method of the presentdisclosure.

Reference is now made to FIGS. 1-2 which illustrate a crystallinesilicon substrate prior to etching and after etching using an etchantsolution in accordance with the present disclosure. Specifically, FIG. 1illustrates a crystalline silicon substrate 10 which has an exposed(100) oriented surface, i.e., face, 12. In one embodiment thecrystalline silicon substrate 10 comprises single-crystalline silicon.The terms “single-crystalline or mono-crystalline silicon” denotes anysilicon substrate in which the crystal lattice of the entire substrateis continuous and unbroken to the edges of the substrate, with no grainboundaries.

When single-crystalline silicon is employed as crystalline siliconsubstrate 10, the single-crystalline silicon can be fabricated usingmethods that are well known in the art. In one embodiment, thesingle-crystalline silicon can be formed by Czocharalski or othertechniques using directional solidification. In another embodiment, thesingle-crystalline silicon can be a thin film that is grown on top of asubstrate (for example, by epitaxial techniques).

In some embodiments, the crystalline silicon substrate 10 is non-doped.In other embodiments, the crystalline silicon substrate 10 is doped witheither a p-type dopant or an n-type dopant. As used throughout thepresent application, “p-type” refers to the addition of impurities to anintrinsic semiconductor that creates deficiencies of valence electrons.Examples of p-type dopants, i.e., impurities, that can be present incrystalline silicon substrate 10 include, but are not limited to, boron,aluminum, gallium and indium. In one embodiment in which the crystallinesilicon substrate 10 includes p-type dopant, the p-type dopant ispresent in a concentration ranging from 1×10⁹ atoms/cm³ to 1×10²°atoms/cm³. In another embodiment in which the crystalline siliconsubstrate 10 includes a p-type dopant, the p-type dopant is present in aconcentration ranging from 1×10¹⁴ atoms/cm³ to 1×10¹⁹ atoms/cm³. As usedthroughout the present application, “n-type” refers to the addition ofimpurities that contributes free electrons to an intrinsicsemiconductor. Examples of n-type dopants, i.e., impurities, that can bepresent in the crystalline silicon substrate 10 include, but are notlimited to, antimony, arsenic and phosphorous. In one embodiment inwhich the crystalline silicon substrate 10 includes an n-type dopant,the n-type dopant is present in a concentration ranging from 1×10⁹atoms/cm³ to 1×10²° atoms/cm³. In another embodiment in which thecrystalline silicon substrate 10 includes an n-type dopant, the n-typedopant is present in a concentration ranging from 1×10¹⁴ atoms/cm³ to1×10¹⁹ atoms/cm³.

In one embodiment, the crystalline silicon substrate 10 of FIG. 1 can bean element of any photovoltaic device, such as, for example, an elementof solar cell. In the case of solar cell applications, the crystallinesilicon substrate 10 needs to have a non-planar surface to improve lightcapture within the crystalline silicon substrate 10.

After providing the crystalline silicon substrate 10, the crystallinesilicon substrate 10 is textured by immersing the same in an etchantsolution of the present application to form a pyramid shaped texturedsurface. FIG. 2 illustrates the resultant structure of FIG. 1 aftertexturing the crystalline silicon substrate 10. In FIG. 2, referencenumeral 10′ denotes the etched crystalline silicon substrate, andreference numeral 14 denotes the pyramid shaped textured surface that isa result of the immersing. The textured surface of the crystallinesilicon substrate has (111) faces that are now exposed.

The etchant solution employed in the present disclosure is an aqueousalkaline etchant solution that includes potassium hydroxide (KOH),sodium hydroxide (NaOH), choline hydroxide, tetramethylammoniumhydroxide (TMAH) or tetraethylammonium hydroxide (TEAH) as an alkalinecomponent. In one embodiment, the alkaline component is present in theaqueous alkaline etchant solution of the present disclosure in aconcentration of from 0.5 weight percent to 5 weight percent. In anotherembodiment, the alkaline component is present in the aqueous alkalineetchant solution of the present disclosure in a concentration of from 1weight percent to 3 weight percent.

The aqueous alkaline etchant solution of the present disclosure alsoincludes a nanoparticle slurry. The term “nanoparticle slurry” as usedthroughout the present disclosure denotes a suspension of colloidalparticles in an aqueous solution. The colloidal particles that can bepresent in the nanoparticle slurry include, but are not limited to, atleast one of alumina, silica, ceria, berylia, magnesia, zirconia andtitania. In one embodiment, the colloidal particles are silicaparticles.

In one embodiment, the colloidal particles that are present in thenanoparticle slurry have a particle size from 10 nm to 1000 nm. Inanother embodiment, the colloidal particles that are present in thenanoparticle slurry have a particles size from 30 nm to 200 nm. Otherparticle sizes can also be used so long as the particle size of thecolloidal particle is below 2500 nm.

In one embodiment, the concentration of the nanoparticle slurry in theaqueous alkaline etchant solution itself, calculated on a dry basis, isfrom 0.1 weight percent to 5 weight percent.

In another embodiment, the concentration of the nanoparticle slurry inthe aqueous alkaline etchant solution itself, calculated on a dry basis,is from 0.3 weight percent to 1.5 weight percent.

In addition to containing an alkaline component and a nanoparticleslurry, the aqueous alkaline etchant solution of the present disclosuremay also contain, as an additive, an alcohol; the alcohol additiverepresents an optional component of the aqueous alkaline etchantsolution of the present disclosure. In one embodiment, the alcohol thatcan be optionally present in the aqueous alkaline etchant solution ofthe present disclosure comprises isopropyl alcohol. In anotherembodiment, the alcohol that can be optionally present in the aqueousalkaline etchant solution of the present disclosure comprises glycerol.In yet another embodiment, the alcohol that can be optionally present inthe aqueous alkaline etchant solution of the present disclosurecomprises ethylene glycol. In addition to the aforementioned alcohols,other alcohols including from 2 to 12 carbon atoms which arestraight-chained, branched or cyclic can also be used. For example,cyclohexanol and ethyl hexanol can also be employed as an additive ofthe aqueous alkaline etchant solution of the present disclosure.

In one embodiment in which an alcohol is present as an additive, thealcohol is present in the aqueous alkaline etchant solution in aconcentration of from 0.1 weight percent to 20 weight percent. Inanother embodiment in which an alcohol is present as an additive, thealcohol is present in the aqueous alkaline etchant solution in aconcentration of from 0.2 weight percent to 10 weight percent.

The aqueous alkaline etchant solution may also include a surfactant. Thesurfactant can optionally be co-used with the alcohol mentioned above,or it can be used by itself, i.e., without any alcohol. The surfactantthat can be used in the present disclosure may include ionic (anionicand cationic) surfactants, zwitterionic surfactants, and/or nonionicsurfactants.

Examples of anionic surfactants include, but are not limited to,sulfates such as alkyl sulfates (e.g., ammonium lauryl sulfate andsodium lauryl sulfate), alkyl ether sulfates (e.g., sodium laurethsulfate and sodium myeth sulfate), sulfonates (e.g., dioctyl sodiumsulfosuccinate), sulfonate fluorosurfactants (e.g.,perfluorooctanesulfonate and perfluorobutanesulfonate), alkyl benzenesulfonates, phosphates such as, for example, alkyl aryl ether phosphateand alkyl ether phosphate, carboxylates such as, for example, alkylcarboxylates (e.g., fatty acids salts and sodium stearate), andcarboxylate fluorosurfactants such as, for example, perfluorononanoateand perfluorooctanoate.

Examples of cationic based surfactants include, but are not limited to,primary, secondary or tertiary amines, and quaternary ammonium compounds(e.g., alkyltrimethylammonium salts, cetylpyridinium chloride,polyethoxylated tallow amine, benzalkonium chloirde, nenzethoniumchloride, dimethyldiocadecylammonium chloride, anddioctadecyldimethylammonium bromide).

Examples of zwitterionic surfactants include primary, secondary ortertairy amines, or quaternary ammonium cations with sulfonates (e.g.,(3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate) orsultaines), carboxylates (i.e., amino acids, imino acids and betaines)or phosphates (e.g., lecithin).

Examples of nonionic surfactants include fatty alcohols (e.g., cetylalcohol, stearyl alcohol, cetostearyl alcohol and oleyl alcohol),polyoxyethylene glycol alkyl ethers (e.g., octaethylene glycolmonododecyl ether and pantaethylene glycol monododecyl ether),polyoxypropylene glycol alkyl ethers, glucoside alkyl ethers (e.g.,decyl glycoside, lauryl glucoside and octyl glucoside, polyoxyethyleneglycol alkylphenol ethers, dodecyldimethylamine oxide, and blockcopolymers of polyethylene glycol and polypropylene glycol.

In some embodiments, ionic liquids, which are salts that are molten atroom temperature, can be employed as the surfactant. Examples of ionicliquids that can be employed in the present disclosure include, but arenot limited to, salts comprised of cationic species, such asimidazolium, phosphonium, and ammonium compounds, associated withanionic species, such as borate, halide, sulfate, acetate, phosphate,and sulfonate compounds.

In one embodiment in which a surfactant is present in the aqueousalkaline etchant solution of the present disclosure, the surfactant ispresent in the aqueous alkaline etchant solution in a concentration offrom 0.00005 weight percent to 0.1 weight percent. In another embodimentin which a surfactant is present, the surfactant is present in theaqueous alkaline etchant solution of the present disclosure in aconcentration of from 0.001 weight percent to 0.01 weight percent.

The aqueous alkaline etchant solution of the present disclosure may alsoinclude pre-etched conditioning silicon. In one embodiment, thepre-etched conditioning silicon may be used together with the alcoholand/or the surfactant mentioned above. In one embodiment, the pre-etchedconditioning silicon is present in the aqueous alkaline etchant solutionin an amount from 0.1 weight percent to 1.5 weight percent. In anotherembodiment of the present disclosure, the pre-etched conditioningsilicon is present in the aqueous alkaline etchant solution in aconcentration of from 0.5 weight percent to 1.0 weight percent.

In accordance with the present disclosure, the remainder of the aqueousalkaline etchant solution of the present disclosure is water (typicallyde-ionized water). As such, water is present in amount such that its sumwith the alkaline component, the nanoparticle slurry, optional alcoholadditive, optional surfactant, and optional pre-etched conditioningsilicon adds up to 100%. In one embodiment, the alkaline component, thenanoparticle slurry, the optional alcohol additive, the optionalsurfactant, the optional pre-etched conditioning silicon, and waterconstituent the entirety of the etchant solution of the presentdisclosure.

The aqueous alkaline etchant solution can be formed by various methods.The above mentioned components can be added in different orders. Forexample and in one embodiment, the etchant solution can be prepared byfirst providing the alkaline component, admixing water to theconcentration for the texturing process, then adding a nanoparticleslurry into the solution at an enhanced temperature, e.g., 70°-90° C.,and thereafter admixing the optional alcohol additive and/or surfactantand/or pre-etched preconditioning silicon. In another embodiment, theetchant solution can be prepared by first providing the alkalinecomponent, admixing a lower amount of water to a higher concentrationthan that needed for the texturing process, then adding the nanoparticleslurry into the solution at an enhanced temperature, e.g., 70°-90° C.,and thereafter admixing more water to dilute to the desiredconcentration for the texturing process, and lastly admixing theoptional alcohol additive and/or surfactant and/or pre-etchedconditioned silicon. In yet another embodiment, water is first providedand then the alkaline and nanoparticle slurry can be added in any order,followed by optional alcohol additive and/or surfactant and/orpre-etched conditioning silicon. During the admixing step, stifling istypically employed.

In one example, the aqueous alkaline etchant solution can include ananoparticle slurry comprising 30 weight percent of silica particleshaving a particle size of 50 nm mixed in dilute KOH, and optionalalcohol additive and/or surfactant.

As stated above, the crystalline silicon substrate 10 is textured byimmersing the same in the aqueous alkaline etchant solution of thepresent disclosure. In one embodiment, the immersing is performed at atemperature from 70° C. to 90° C. In another embodiment, the immersingis performed at a temperature from 75° C. to 85° C. The term “immersing”denotes that at least surface 12 of the crystalline silicon substrate 10is placed into a bath that includes the aqueous alkaline etchantsolution mentioned above.

In one embodiment, the duration of the immersing of the crystallinesilicon substrate 10 into the aqueous etchant solution is from 10minutes to 60 minutes. In another embodiment, the duration of theimmersing of the crystalline silicon substrate 10 into the aqueousetchant solution is from 15 minutes to 30 minutes.

When the above conditions are employed for texturing a crystallinesilicon substrate 10, the aqueous alkaline etchant solution of thepresent disclosure typically has an etch rate from 0.1 μm/min to 1.0μm/min. More typically, the aqueous alkaline etchant solution of thepresent disclosure has an etch rate from 0.30 μm/min to 0.50 μm/min.

Additional texturing can be performed as deemed necessary by repeatingthe immersing step mentioned above.

In one embodiment of the present disclosure, the textured crystallinesilicon substrate has a weighted average reflectance between 400 nm to1100 nm of from 0.09 to 0.13. In another embodiment of the presentdisclosure, the textured crystalline silicon substrate has a weightedaverage reflectance between 400 nm to 1100 nm of from 0.10 to 0.12. Theterm “weighted average reflectance” is used throughout the presentapplication to denote the average reflectance weighted to the photonflux density of an AM 1.5 G spectrum. The weighted average reflectancecan be determined by reflectance spectroscopy.

As mentioned above the aqueous alkaline etchant solution of the presentdisclosure leads to a substantially improved textured crystalline Sisurface, with an increased density of smaller-sized pyramids compared toa KOH-only process. By “improved textured crystalline Si surface” it ismeant an increase of the surface area that is covered by pyramids. By“increased density” it is meant an increase of the number of pyramids ina unit area.

The improved morphology of the textured surface that is achieved by thepresent disclosure leads to a significant improvement in the measuredreflectance. For example, a crystalline Si surface textured with aKOH-only bath is twice as reflective as a similar surface textured bythe method of the present disclosure. In some embodiments, the totalloss of Si that may occur using the method of the present disclosure maybe significantly reduced compared to KOH-only texturing.

The following examples are provided to illustrate some embodiments ofthe present disclosure and to also illustrate some of the abovementioned advantages that can be achieved using the aqueous alkalineetchant solution of the present disclosure to texture a surface of asingle-crystalline silicon substrate.

Example 1

In this example, which represents an embodiment of the presentdisclosure, a surface of a saw damaged etched (SDE) single-crystallinesilicon substrate was textured by immersing the substrate in an aqueousalkaline etchant solution including a nanoparticle slurry. Specifically,the aqueous alkaline etchant solution employed in texturing the surfaceof the SDE single-crystalline silicon substrate of this examplecontained 1.5 weight % KOH and 1.2 weight % of a nanoparticle slurrycontaining 30 weight % of silica particles having a size of ˜50 nmdispersed in water (0.4 weight % of silica particles on a dry basis inthe aqueous alkaline etchant solution). The texturing was performed at75° C., for 15 minutes. The etching rate was 0.41 μm/min, and theweighted average reflectance WAR was 0.112. FIG. 3 shows the actual SEMphotograph of the textured SDE single-crystalline silicon wafer of thisexample.

Example 2

In this example, which represents another embodiment of the presentdisclosure, a surface of an as-cut single-crystalline silicon substratewas textured by immersing the substrate in an aqueous alkaline etchantsolution including a nanoparticle slurry. Specifically, the aqueousalkaline etchant solution employed in texturing the surface of the SDEsingle-crystalline silicon substrate of this example contained 1.5weight % KOH and 1.2 weight % of a nanoparticle slurry containing 30weight % of silicon particles having a size of ˜50 nm dispersed in water(0.4 weight % of silica particles on a dry basis in the aqueous alkalineetchant solution). The texturing was performed at 75° C., for 30minutes. The etching rate was 0.45 μm/min, and the weighted averagereflectance WAR was 0.114. FIG. 4 shows the actual SEM photograph of thetextured SDE single-crystalline silicon wafer of this example.

Example 3

In this example, the weighted average reflectance of the texturedsingle-crystalline silicon substrates of Examples 1 and 2 were comparedwith the weighted average reflectance of a non-textured as-cutsingle-crystalline silicon substrate and a non-textured saw damagedetched (SDE) single-crystalline silicon substrate. The results aresummarized in Table 1.

TABLE 1 Weighted Average Sample Reflectance (WAR) Non-textured As-Cutsingle-crystalline silicon 0.258 substrate Non-textured SDEsingle-crystalline silicon 0.397 substrate Textured single-crystallinesilicon substrate of 0.112 Example 1 Textured single-crystalline siliconsubstrate of 0.114 Example 1

The data shown in Table 1 illustrates that textured samples of thepresent disclosure all had a lower weighted average reflectance thannon-textured samples.

Comparative Example 1

In this comparative example, a surface of a saw damaged etched (SDE)single-crystalline silicon substrate was textured by immersing thesubstrate in a prior art aqueous alkaline etchant solution.Specifically, the aqueous alkaline etchant solution employed intexturing the surface of the SDE single-crystalline silicon substrate ofthis comparative example contained 1.5 weight % KOH and 0.5 weight % ofa pre-etched conditioning silicon. The texturing was performed at 75°C., for 15 minutes. The etching rate was 0.28 μm/min, and the weightedaverage reflectance WAR was 0.117. FIG. 5 shows the actual SEMphotograph of the textured SDE single-crystalline silicon wafer of thiscomparative example.

Comparative Example 2

In this comparative example, a surface of a saw damaged etched (SDE)single-crystalline silicon substrate was textured by immersing thesubstrate in a prior art aqueous alkaline etchant solution.Specifically, the aqueous alkaline etchant solution employed intexturing the surface of the SDE single-crystalline silicon substrate ofthis comparative example contained 1.5 weight % KOH. The texturing wasperformed at 75° C., for 15 minutes. The etching rate was 0.73 μm/min,and the weighted average reflectance WAR was 0.268. FIG. 6 shows theactual SEM photograph of the textured SDE single-crystalline siliconwafer of this comparative example.

Example 4

In this example, the weighted average reflectance of the texturedsingle-crystalline silicon substrate of Example 1 was compared with theweighted average reflectance of the textured single-crystalline siliconsubstrates of Comparative Examples 1 and 2. The results are summarizedin Table 2.

TABLE 2 Weighted Average Sample Reflectance (WAR) Texturedsingle-crystalline silicon substrate of 0.112 Example 1 Texturedsingle-crystalline silicon substrate of 0.117 Comparative Example 1Textured single-crystalline silicon substrate of 0.268 ComparativeExample 1

The data shown in Table 2 illustrates that the textured sample of thepresent disclosure had a lower weighted average reflectance than thesample that was textured in a KOH-only solution, and a comparable, ifnot lower, reflectance to the sample that was textured in asilicon-conditioned KOH solution.

While the present disclosure has been particularly shown and describedwith respect to various embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A method of forming a textured silicon surface, said methodcomprising immersing a crystalline silicon substrate into an aqueousalkaline etchant solution to form a pyramid shaped textured surface,with (111) faces exposed, on said crystalline silicon substrate, whereinthe aqueous alkaline etchant solution comprises an alkaline componentand a nanoparticle slurry.
 2. The method of claim 1, wherein saidalkaline component comprises potassium hydroxide (KOH), sodium hydroxide(NaOH), choline hydroxide, tetramethylammonium hydroxide (TMAH) ortetraethylammonium hydroxide (TEAH).
 3. The method of claim 2, whereinsaid alkaline component is KOH.
 4. The method of claim 1, wherein saidalkaline component is present in said aqueous alkaline etchant solutionin a concentration of from 0.5 weight percent to 5 weight percent. 5.The method of claim 1, wherein said nanoparticle slurry comprisescolloidal particles of at least one of alumina, silica, ceria, berylia,magnesia, zirconia and titania.
 6. The method of claim 5, wherein saidnanoparticle slurry comprises silica particles.
 7. The method of claim1, wherein said nanoparticle slurry has colloidal particles of a sizefrom 10 nm to 1000 nm.
 8. The method of claim 1, wherein saidnanoparticle slurry is present in said aqueous alkaline etchant solutionin a concentration of from 0.1 weight percent to 5 weight percent on adry basis.
 9. The method of claim 1, wherein said aqueous alkalineetchant solution further comprises at least one additive, said at leastone additive is selected from an alcohol.
 10. The method of claim 9,wherein said alcohol is isopropyl alcohol, ethylene glycol, glycerol,cyclohexanol or ethyl hexanol.
 11. The method of claim 9, wherein saidalcohol is present in said aqueous alkaline etchant solution in aconcentration from 0.1 weight percent to 20 weight percent.
 12. Themethod of claim 1, wherein said aqueous alkaline etchant solutionfurther comprises a surfactant.
 13. The method of claim 12, wherein saidsurfactant is present in said aqueous alkaline etchant solution in aconcentration from 0.00005 weight percent to 0.1 weight percent.
 14. Themethod of claim 1, wherein said aqueous alkaline etchant solutionfurther comprises pre-etched conditioning silicon.
 15. The method ofclaim 14, wherein said pre-etched conditioning silicon is present insaid aqueous alkaline etchant solution in a concentration from 0.1weight percent to 1.5 weight percent.
 16. The method of claim 1, whereinsaid immersing is performed at a temperature from 70° C. to 90° C. 17.The method of claim 1, wherein said crystalline silicon substrate has a(100) oriented surface prior to immersing.
 18. The method of claim 1,wherein said crystalline silicon substrate is a component of a solarcell.
 19. The method of claim 1, wherein said crystalline siliconsubstrate comprises single-crystalline silicon.
 20. An aqueous alkalineetchant solution for texturing a surface of a crystalline siliconsubstrate, said aqueous alkaline etchant solution comprising 0.5 weightpercent to 5 weight percent of an alkaline component and from 0.1 weightpercent to 5 weight percent of a nanoparticle slurry on a dry basis. 21.The aqueous alkaline etchant solution of claim 20, alkaline componentcomprises potassium hydroxide (KOH), sodium hydroxide (NaOH), cholinehydroxide, tetramethylammonium hydroxide (TMAH) or tetraethylammoniumhydroxide (TEAH).
 22. The aqueous alkaline etchant solution of claim 21,wherein said alkaline component is KOH.
 23. The aqueous alkaline etchantsolution of claim 20, wherein said nanoparticle slurry comprisescolloidal particles of at least one of alumina, silica, ceria, ceria,magnesia, zirconia and titania.
 24. The aqueous alkaline etchantsolution of claim 23, wherein said nanoparticle slurry comprises silicaparticles.
 25. The aqueous alkaline etchant solution of claim 20,wherein said nanoparticle slurry has colloidal particles of a size from10 nm to 1000 nm.
 26. The aqueous alkaline etchant solution of claim 20,further comprises from 0.1 weight percent to 20 weight percent of atleast one alcohol, said at least one alcohol is selected from isopropylalcohol, ethylene glycol, glycerol, cyclohexanol and ethyl hexanol.